Hydroformylation is one of the most important reactions in industrial scale chemistry, having an annual global production capacity of several million tonnes. This involves reacting alkenes (olefins) with a mixture of carbon monoxide and hydrogen (also: synthesis gas or syngas) using a catalyst to give aldehydes, which are important and valuable intermediates in the production of chemical bulk products such as alcohols, esters or plasticizers.
Hydroformylation is conducted exclusively under homogeneous catalysis on the industrial scale. The soluble transition metal catalyst systems are typically based on cobalt or rhodium, which is often used together with phosphorus-containing ligands, for example phosphines or phosphites, for the hydroformylation of comparatively short-chain olefins.
There are various problems in the known processes, and these are especially linked to the fact that both rhodium and cobalt and compounds thereof are comparatively costly. There is a high level of energy expenditure and complex chemical engineering in order to very substantially avoid losses of catalyst during the hydroformylation process, for example by catalyst recycling steps, some of them very complex. Moreover, product purification steps are becoming more complex in order to ensure that as far as possible no catalyst residues remain in the product.
Further problems with the known homogeneously catalysed processes are the stability of the ligands, which have to withstand the hydroformylation conditions, such as temperature, pressure, pH etc., and consumption of the solvent used during the process, which can be compensated for by replenishment.
In order to get round the aforementioned problems in the homogeneously catalysed hydroformylation, there has been development of hydroformylation methods in which the catalyst is heterogenized, especially by immobilization on a support material (cf. introductory discussion in WO 2015/028284 A1). The terms “heterogenization” and “immobilization” should accordingly be understood such that the catalyst is immobilized by formation of a thin liquid film with the aid of an ionic liquid on the surface and/or in the pores of a solid support material and there is no reaction solution in the conventional sense in which the catalyst is homogeneously dissolved.
With regard to the immobilization/heterogenization, the already mentioned WO 2015/028284 A1 discloses what are called SILP systems (SILP=Supported Ionic Liquid Phase), in which the catalyst system is immobilized with rhodium, iridium or cobalt as central atom, especially on a porous silicon dioxide support using an ionic liquid.
However, the problem with the known SILP systems is that, after a certain service life, a distinct decreasing catalyst activity and hence a reduction in conversion can be observed. This may be attributable to various effects, for example condensation of the products in the pores and corresponding further reactions such as aldol condensations, or the formation of water that can lead to deactivation of the ligands, the formation of by-products and/or the flooding of the pores, as a result of which the catalyst may be discharged.
The problem addressed by the present invention was therefore that of providing a process for hydroformylating olefins that does not have the aforementioned problems and especially leads to an increase in the conversion and lifetime of the catalyst.